Device and method for combined purification and compression of hydrogen containing CO and the use thereof in fuel cell assemblies

ABSTRACT

The invention relates to a device for combined purification and compression of hydrogen containing CO. Said device comprises a polymer electrolyte membrane (PEM) cell with a proton-conducting polymer membrane ( 3 ), an anode on one side of the membrane and a cathode ( 4 ) on the other side thereof. Said anode is in the form of a two-layer anode ( 5 ) which comprises a CO-oxidation-selective catalytic layer ( 1 ) on the side facing away from the membrane and an electrochemically active layer ( 2 ) for oxidation of the hydrogen to form protons on the side facing the membrane. The PEM cell also has a flat, porous gas distribution layer provided on the anode side and attached to a current collector. Consequently, the PEM cell resists differential pressures of at least approximately 10 bar. The invention also relates to a process for combined purification and compression of hydrogen containing CO using said device. The CO in the hydrogen containing CO is oxidized selectively to form CO 2  in the PEM cell, and the hydrogen subsequently extensively free of CO is compressed by the PEM cell by electrochemical means at differential pressures of at least approximately 10 bar. The resultant hydrogen is suitable as fuel in PEM fuel cells for vehicles.

DESCRIPTION

The invention relates to an apparatus and a process for the combinedpurification and compression of CO-containing hydrogen and to the use ofthe hydrogen thus obtained as a fuel gas in polymer electrolyte membrane(PEM) fuel cells.

Fuel cells are distinguished by high electric efficiency, low pollutantemissions and low noise levels and are therefore highly suitable todrive vehicles. Finding favour for this application is the polymerelectrolyte membrane (PEM) fuel cell, given its operating temperaturerange, its robustness and its tolerance with respect to pressurefluctuations. PEM fuel cells do, however, have to be operated with purehydrogen (H₂) as a fuel. On the other hand, it is advantageous that, asan alternative to oxygen (O₂), air can be used as the oxidant. COfractions above 20 ppm in the hydrogen used as a fuel gas causepoisoning of the electrode and a marked drop in performance inconjunction with excessive specific energy consumption.

The purification of hydrogen conventionally takes place discontinuouslyby means of pressure swing adsorption (PSA). The processes based on thistechnique do however involve high specific energy consumption, require abulky construction and are therefore unsuitable for operating vehicles.

Known mechanical processes for compressing hydrogen comprise, forexample, the use of piston compressors, screw compressors and rotarycompressors. Such methods do, however, involve high noise levels and,particularly in the case of installations carrying an electric load ofup to 10 kW, require a high specific energy input.

GB-A-2 268 322 discloses that pure hydrogen can be compressedelectrochemically, using PEM cells, to up to a differential pressure of5 bar. Above such a differential pressure the electrode mountings of theknown PEM cells are subject to cracking.

Hydrogen produced as a reformer gas from methanol or other hydrocarbonscontains varying amounts of carbon monoxide (CO). The provision ofultrapure, CO-free hydrogen from such reformer gases, in particular forthe cold-start phase of fuel cell vehicles, while at the same timeensuring a load change behaviour on a timescale ranging frommilliseconds to seconds, is technically not feasible as yet.

The object of the present invention is therefore to provide a low-noiseand energy-efficient apparatus and a process for the combinedpurification and compression of CO-containing hydrogen, differentialpressures of at least about 10 bar being achievable.

This object is achieved according to the invention by an apparatusaccording to Claim 1 and a process according to Claim 9. Advantageous orparticularly expedient refinements of the subject-matter of theinvention are specified in the subordinate claims.

The invention therefore relates to an apparatus for the combinedpurification and compression of CO-containing hydrogen, comprising apolymer-electrolyte membrane-(PEM)-cell including a proton-conductingpolymer membrane, an anode on one side and a cathode on the other sideof the membrane, the anode being in the form of a bilayer anode whichcomprises a CO-oxidation-selective catalyst layer on the side facingaway from the membrane and an electrochemically active layer foroxidizing the hydrogen to protons on the side facing the membrane, andfurther including a planar, porous support layer which is provided atthe anode side and adjoins a current collector, so that the PEM cell isable to withstand differential pressures of at least about 10 bar.

The invention further relates to a process for the combined purificationand compression of CO-containing hydrogen by means of an apparatusaccording to the invention, wherein the CO in the CO-containing hydrogenis oxidized selectively to CO₂ in the first PEM cell, by means of thehydrogen being admixed with an amount of oxygen corresponding to the COcontent, said oxygen catalytically oxidizing the CO on theCO-oxidation-selective catalyst layer of the bilayer anode to give CO₂,and wherein the now largely CO-free hydrogen is compressedelectrochemically by the PEM cell to differential pressures of at leastabout 10 bar, by virtue of the electrochemically active layer of thebilayer anode oxidizing the hydrogen to protons which, having passed themembrane at the cathode side, are discharged under pressure to giveultrapure hydrogen.

Finally, the invention relates to the use of a hydrogen obtained inaccordance with the process according to the invention as a fuel gas inPEM fuel cells, especially for vehicles.

The invention is explained in more detail with reference to theaccompanying drawing in which

FIG. 1 shows the schematic construction of a bilayer anode usedaccording to the invention in a composite arrangement with a polymermembrane and a cathode.

According to the invention it was found that the purification ofCO-containing hydrogen gas can be achieved in a simple manner by theelectrochemical hydrogen compression making use of a PEM cell whichincludes an electrode in the form of a bilayer anode, one layer of thebilayer anode acting catalytically to oxidize CO present in the hydrogengas selectively to CO₂ by means of admixed oxygen. Depending on the COfraction, the hydrogen gas may, for example, be admixed with up to 1000ppm of oxygen. This purification effect reduces the CO content of thehydrogen to such an extent that the second layer of the bilayer anode,i.e. the electrode layer exhibiting electrochemical action, is notpoisoned by CO. The second electrode layer is therefore able to act as aconventional PEM anode and to oxidize the now largely CO-free hydrogento protons which, having passed the polymer membrane, are discharged atthe cathode under pressure to give ultrapure hydrogen. Thus the PEM cellacts as a continuous CO getter.

By means of the apparatus according to the invention it is possible tocompress the pure hydrogen gas to at least about 10 bar. The cracking,to which the electrode mountings in conventional PEM cells are subjectabove a differential pressure of about 5 bar, is prevented according tothe invention by appropriate reinforcements on the anode side of the PEMcell. This is achieved by the electrode-membrane-electrode unit beingsupported on the anode side (low-pressure side or hydrogen gas supplyside), by means of a planar, low-roughness gas distribution layer, insuch a way that no unevennesses caused by cavities, such as gasdistribution ducts, are produced which might cause rupturing of themembrane at high differential pressures. This gas distribution layeradjoins a planar or smooth current collector plate.

According to one embodiment, the current collector plate has no gasdistribution ducts and is provided with the gas distribution layer whichhas a resistance to pressure of at least the sum of the applicationpressure and the particular final hydrogen pressure and has a smooth orplanar surface. According to another embodiment, the current collectorplate has gas distribution ducts on the surface side and is providedwith the gas distribution layer which has a resistance to pressure of atleast the sum of the application pressure and the particular finalhydrogen pressure and has an adequate flexural strength so that theapplication pressure of the membrane is evenly distributed,notwithstanding the gas ducts present, over the entire area. The gasdistribution layer can be formed, for example, from metal fibre platesor metal felt plates.

The CO-oxidation-selective catalyst layer of the bilayer anode suitablycomprises an electrically conducting matrix and an oxidic supportmaterial, which is coated with fine, metallic, CO-oxidation-selectivecatalyst particles. Examples of suitable materials for the electricallyconducting matrix are metal felt or graphite.

The oxidic support material enhances the catalytic action (spillovereffect) and is formed, for example, from zirconium oxide, titaniumdioxide, cobalt oxide, cerium oxide, praseodymium oxide, yttrium oxide,mixtures thereof or mixed oxides thereof. The specific surface area ofthese oxidic support materials is suitably from 1 to 100 m²/g,preferably more than 10 m²/g.

The CO-oxidation-selective catalyst particles are formed, for example,from gold, ruthenium, rhodium or alloys thereof.

The second electrode layer, exhibiting electrochemical action, of thebilayer anode is formed from such materials as are customarily used forPEM anodes.

The apparatus according to the invention may comprise a single PEM cell.To achieve differential pressures of more than about 15 bar, however, asecond PEM cell is preferably connected in series with the first PEMcell. This second and, where present, each further PEM cell comprises aproton-conducting polymer membrane, an anode on one side and a cathodeon the other side of the membrane, and a planar, porous gas distributionlayer which is provided at the anode side and adjoins a currentcollector plate, so that the said cell is able to withstand adifferential pressure of more than about 15 bar.

The proton-conducting polymer membrane employed in the apparatusaccording to the invention is made of materials customarily employed forPEM cells, such as are available, for example, under the trademarkNafion. This polymer membrane is provided on both sides, in a mannerknown per se, with one electrode each which suitably comprise acatalytically activated support substrate and binders, for examplecarbon black and polytetrafluoroethylene. Potentially suitable as thecatalyst are materials known per se which exhibit sufficiently highactivity for hydrogen oxidation or hydrogen reduction, preferably metalsand alloys of the platinum group.

Control of the compression efficiency of the PEM cells is suitablyachieved by means of a control signal. For this purpose, the voltagesource that provides the current required for compression isdisconnected periodically for a brief interval and the potential whichestablishes itself at the end or during the disconnection time ismeasured. This potential allows a determination of the actual pressureaccording to the Nernst equation. Via a conventional control device,this potential is compared with the potential that establishes itself atthe target pressure, and based on the difference either the cell currentor the cell voltage is preset. The control method used is not subject toany particular restriction, but preference is given to an on/off controlor a PID control method.

The PID (Proportional-Integral-Differential) method is a known controlmethod and comprises setting of the manipulated variable additively from(i) a signal proportional to the difference between controlled variableand reference variable and (ii) from an integral signal formed from (i)and (iii) from a differential signal formed from (i). Favourabletransient response of the control can be achieved by suitably settingthe signal components, which result from (i), (ii) and (iii), to themanipulated variable.

The design of a bilayer anode employed according to the invention isshown in FIG. 1 by way of example, 5 indicating the bilayer anode, 1 theCO-oxidation-selective catalyst layer and 2 the electrochemically activelayer for the oxidation of the hydrogen to protons. The catalyst layer 1here consists of an electrically conducting metal felt matrix 6, whichholds an oxidic support material 7 which enhances the catalytic effectand has a specific surface area of more than about 10 m²/g. The supportmaterial in turn is coated with very finely distributed metalliccatalyst particles 8. The electrochemically active layer 2 consists, forexample, of a platinum/ruthenium thin-film anode. The bilayer anode 5 isin a composite arrangement with a polymer membrane 3 and aplatinum/carbon cathode 4.

What is claimed is:
 1. Apparatus for combined purification andcompression of CO-containing hydrogen, comprising a polymer electrolytemembrane (PEM) cell including a proton-conducting polymer membrane (3),an anode on one side and a cathode (4) on the other side of themembrane, the anode having a first side facing away from the membraneand a second side facing the membrane, the anode being in the form of abilayer anode (5) which comprises a CO-oxidation-selective catalystlayer (1) on the side facing away from the membrane and anelectrochemically active layer (2) for oxidizing the hydrogen to protonson the side facing the membrane, wherein the CO-oxidation-selectivecatalyst layer (1) of the bilayer anode (5) comprises an electricallyconducting metal felt matrix (6) and an oxidic support material (7)which is coated with metallic CO-oxidation-selective catalyst particles(8) selected from the group consisting of gold, ruthenium, rhodium andalloys thereof, and further including a planar, porous gas distributionlayer which is provided at the anode side and adjoins a currentcollector, so that the PEM cell is able to withstand differentialpressures of at least about 10 bar.
 2. Apparatus according to claim 1,comprising at least one second PEM cell connected in series with thefirst PEM cell, the second PEM cell comprising a proton-conductingpolymer membrane, an anode on one side and a cathode on the other sideof the membrane, and a planar, porous gas distribution layer which isprovided at the anode side and adjoins a current collector, so that thesecond PEM cell is able to withstand a differential pressure of morethan about 15 bar.
 3. Apparatus according to claim 1, wherein thecurrent collectors on the anode side of the PEM cells have a planarsurface without gas distribution ducts and the gas distribution layerhas a resistance to pressure of at least the sum of an initial pressureand the final hydrogen pressure.
 4. Apparatus according to claim 1,wherein the gas distribution layer is formed from a metal fibre plate.5. Apparatus according to claim 1, further comprising a means forcontrolling the hydrogen pressure of the PEM cell by means of a controlsignal which is generated at regular intervals by disconnection of thevoltage source of the cell and determination of the deviations of thecell voltages then measured from the voltage following from the Nernstequation.
 6. Apparatus according to claim 5, wherein the control signalis formed by an on/off or a PID method.
 7. Apparatus according to claim2, wherein the current collectors on the anode side of the PEM cellshave a planar surface without gas distribution ducts and the gasdistribution layer has a resistance to pressure of at least the sum ofthe CO-containing hydrogen pressure and the purified hydrogen pressure.8. Process for the combined purification and compression ofCO-containing hydrogen by means of the apparatus according to claim 1,wherein the CO in the CO-containing hydrogen is oxidized selectively toCO₂ in the PEM cell, by means of the hydrogen being admixed with anamount of oxygen corresponding to the CO content, said oxygencatalytically oxidizing the CO on the CO-oxidation-selective catalystlayer of the bilayer anode to give CO₂ and hydrogen, and wherein thehydrogen is compressed electrochemically by the PEM cell to differentialpressures of at least about 10 bar, by virtue of the electrochemicallyactive layer of the bilayer anode oxidizing the hydrogen to protonswhich, having passed the membrane at the cathode side, are dischargedunder pressure to give purified hydrogen.
 9. Process according to claim8, wherein the CO-containing hydrogen used is a methanol reformer gas.10. The process according to claim 8, further comprising the step ofutilizing the hydrogen obtained in accordance with the process as a fuelgas in PEM fuel cells.